1. Field of the Invention
This invention relates to spark plugs and, in particular, to materials for a spark plug.
2. Prior Art
Prolonging the life of spark plugs is desirable for a variety of reasons including meeting government regulations, reducing cost, minimizing maintenance and improving reliability. Typically, current production spark plugs have an expected life of about 30,000 miles, One of the major limiting factors of spark plug life is the deterioration of the center electrode. Attempts to overcome this limitation have included developing platinum-tipped center electrodes. For example, a thin platinum pin can be inserted in the ceramic insulator before sintering. Alternatively, a small platinum disk can be recessed into the turned down tip of a conventional electrode and held in place by spot welding. However, using noble metals, such as platinum, is costly.
The most common materials used for production of center electrodes are nickel alloys such as Inconel (nickel-iron-chromium). These alloys have proven to be adequate to meet durability requirements of about 30,000 miles. A modest degree of durability improvement can be achieved with copper cored center electrodes using a nickel alloy as the cladding material.
There are believed to be two basic mechanisms that contribute to the operational deterioration of the center electrode: (1) spark erosion and (2) chemical corrosion. The relationship of each particular type of deterioration, i.e. spark erosion or chemical attack, in relation to temperature is shown in FIG. 2. Although spark erosion appears to be operative at all temperatures, chemical corrosion appears to be a major contributor at normal operating temperatures.
A study of nickel alloy central electrodes indicates that both chromium oxide and chromium sulfide are formed on the surface of the electrode during normal engine operation. These materials are only weakly bonded to the surface and easily removed. This produces fresh surfaces on the nickel alloy electrode that are again susceptible to further chemical corrosion.
Also known are various coatings used to protect gas turbine super alloy components against oxidation and sulfadation. In particular, a family of alloys called M-CrAlY has been developed,
______________________________________ where M = Ni (Nickel), or Co (Cobalt), or Fe (Iron) or combinations of Nickel, Cobalt and Iron, such as NiCo, FeCo, etc. Cr = 15-30 Wt. % Chromium Al = 5-15 Wt. % Aluminum Y = 0-2 Wt. % Yttrium or other active elements e.g. Zr (Zirconium) Hf (Hafnium) Ti (Titanium) ______________________________________
A discussion of the relationship of these alloys to protecting gas turbine super alloy components is in:
(1) Boone, D. H.; Strangman, T. E.; Wilson, L. W.; "Some Effects of Structure and Composition on the Properties of Electron Beam Vapor Deposited Coatings for Gas Turbine Superalloys", J. Vac. Sci. Technol., 11 (4) 641, 1974 and (2) Strangman, T. E.; Hopkins, S. W.; "Thermal Fatigue of Coated Superalloys", Bul. Am. Ceram. Soc. 55, (3) 305, 1976. In this literature, it is taught that the aluminum in the alloys forms a protective scale of Al.sub.2 O.sub.3 and that the Yttrium provides a strong bond between the Al.sub.2 O.sub.3 and the metal electrode substrate.
Although the above techniques and structures are known, there still remains a need for a spark plug with a prolonged life. In particular, it would be desirable to have a spark plug with a durable central electrode. These are some of the problems this invention overcomes.